The original poster hasn't responded to any of these suggestions, lots of them good!
I don't think a radiant barrier placed on the inside of the roof will accomplish much. Keeping the heat out of the massive stone walls and roof is the primary goal. Whitewash is a very inexpensive way to start, I just don't know if it will "stick" to the roof tiles. It actually reacts with masonry to chemically bond to the surface, so if it will, it is a durable as well as inexpensive way to increase reflectance. Lime + water and a pinch of salt is all that's needed to make whitewash. We whitewashed our stucco walls in Wisconsin and it did a great job.
That's not enough though. Inside this stone house the ceiling appears to be the gabled roof. I would install a flat ceiling at whatever height, be sure the attic area above it is well vented, and insulate the new ceiling with 30-50cm of loose fill insulation. This is the single biggest improvement that could be made to reduce overheating, and to reduce the winter heating need as well.
If it is possible, insulation on the outside of the west, then the south, then the east walls would be the most productive order in which to proceed for improving interior living conditions. Likely to cost more than the new interior ceiling with insulation above it.